1. Field of Endeavor
The present invention relates to a burner for a gas turbine and a method for locally cooling the hot gases flow passing through a burner.
In particular the present invention relates to a sequential combustion gas turbine, i.e., a gas turbine having a compressor compressing a main air flow that is fed to a first burner; in the first burner the fuel is injected into the main air flow to form a first mixture that is then combusted in a first combustion chamber and expanded in a high pressure turbine.
Afterwards, the hot gases coming from the high pressure turbine are fed to a second burner where further fuel is injected to form a second mixture. This second mixture is then fed to a second combustion chamber where it is combusted to be then expanded in a low pressure turbine.
The present invention in particular relates to the second burner of the sequential combustion gas turbine.
2. Brief Description of the Related Art
Traditional burners have a duct containing typically four vortex generators and a lance provided with nozzles that inject the fuel.
During operation, while the hot gas flow passes through the duct, the fuel is injected such that it mixes with the hot gases to form the mixture to be combusted.
Moreover, due to the high temperature of the hot gases flowing in the duct, the walls of the duct, vortex generators, lance, etc, are provided with a plurality of small, closely-spaced, uniformly distributed holes from which cooling air is injected.
This cooling air cools the walls of the duct, vortex generators, etc, to guarantee their reliability.
In these burners (second burners of sequential combustion gas turbines) combustion is spontaneous, i.e., after a so-called ignition delay time from injection in the hot gases flow, the fuel autoignites.
In order to have a correct operation, the delay time must be long enough to let the fuel mix with the hot gases and go through the entire burner, entering the combustion chamber.
Nevertheless, in order to increase the efficiency of the gas turbines, the flame temperature in the second combustion chamber and the temperature of the hot gases coming from the high pressure turbine and entering the second burner should be increased.
This causes the delay time to be reduced and the risk for flashback to be increased.
Existing burners have a flashback margin (which within certain limits gives the possibility to decrease the ignition delay time without flashback occurring) that lets the flame and inlet temperatures be increased; nevertheless, in the zones of the core of the vortices (where the flow has a significantly lower axial velocity and thus the fuel has a longer residence time in the burner) the flashback margin is locally too low to guarantee a safe and reliable operation of the gas turbines.